Electric explosion initiator



March 24, 1953 MCFARLAND 2,632,389

ELECTRIC EXPLOSION INITIATOR Filed Sept. 13, 1949 DAVID M. MOFARLANDINVENTOR.

Patented Mar. 24, 1953 ELECTRIC EXPLOSION INITIATOR David M. McFarland,West Chester, Pa., assignor to Atlas Powder Company, Wilmington, Del., acorporation of Delaware Application September 13, 1949, Serial No.115,439

3 Claims. 1

The present invention relates to electric explosion initiators.

An object-of the invention is the provision of an improved electricexplosion initiator which is resistant to static electric discharges.

Another object of the invention is the provision of an electricexplosion initiator which is resistant to ground or stray currents.

A further object of the invention is the provision of an electricexplosion initiator the firing time of which may be readily determined.

Still another object of the invention is the provision of an electricexplosion initiator which may be readily tested for grounded firingcircuits.

A still further object of the invention is the provision of a method fordetermining time of firing of an electric explosion initiator.

Other objects of the invention will be apparent from the followingdescription.

Electric explosion initiators, such as electric blasting caps, usuallyinclude a metal shell into which is charged an explosive material, afiring circuit insulated from the shell and. including insulated legwires and a bridge wire, and an ignition composition in contact with atleast the bridge wire portion of the firing circuit. In operation of theinitiators an electric current is passed through the bridge Wire andheats the bridge wire to a point where an ignition composition fires andthe initiator is exploded.

One type of firing circuit which has found considerable use includesthe' electric match. An electric match customarily includes twoelongated metal terminal sheets separated, by insu lating materialandelectrically connected by means of a bridge wire. The terminal sheetsform tabs to which leg wires are electrically connected, and a matchhead is formed around the bridge wire portion'of the match by coating itwith ignition composition.

The bridge plug type of firing circuit is another one which is oftenemployed. Bridge plug circuits comprise two insulated leg wires leadingthrough a spacing plug and connected'by a bridge wire. The bridge wireis then either enclosed by a loose ignition composition or by a driedpaste. Y

For some time it has been recognized that electric explosion initiatorsmay explode prematurely ifvsufiicient static electric charges accumulateand pass between the firing circuit and the initiator shell through theignition composition.

It has been proposed to protect electric exwire which is grounded to theshell.

plosion initiators from static. electric discharge between firingcircuit and shell by grounding one side of the firing circuit inside theinitiator to the shell. Although this construction may provide excellentresistance against static .discharges it is subject to a very seriousobjectionin that, since each initiator shell is grounded, it provides aperfect arrangement for current leakage from shell to shell through theearth when a number of initiators are connected in a series. Currentleakage will cause misfires of such initiators as do not receive. asufiicient proportion of the firing current. Misfires are of course veryobjectionable in blasting operations and. are dangerous to remedy oncethey have occurred.

Ground or stray current protection is usually obtained by shuntingthefree ends or initiator leg wires until theyare to be fired. Thisprotection is not complete in the case of grounded leg wires. Accordingto the present invention an electric explosion initiator is providedwhich con: tains, in addition to a firing circuit, a third This thirdwire may be connected to the firing circuit at a point outside theinitiator and preferably shunted with the leg wires of thefiring'circuit.

The invention will ,be further described in connection with the drawingswhich illustrate a specific embodiment and in which:

Figure l is a side view, partly in section, of an electric blasting capof the invention, and

Figure 2 is an end view of the shunted wires of the cap of Figure l. 3 i

11 represents a metal shell into the bottom of which are pressed basecharge l3, pri m a1jy detonator charge [5, and flash-charge ll.- Theparticular composition and number ofthese charges is no part of thepresent invention, but; for purpose of illustration it may be statedthat base charge 13 may be tetryl, the primary detonator charge [5 maybe nitromannite, and the flash charge i? may be a mixture ofnitromannite and diazo di-nitrophenol. is represents generally aconventional electric match made up of insulating sheet 2% on each sideof which are affixed metal terminal sheets 23 and 2 5. 2! represents-anignitioncompos-ition, such as for example, a-nitrocellulose lacquercontain-- ing copper acetylide surrounding the lower por? tion ofterminal sheets% and 25 and agbridge wire connecting them (not shown).29 rep: resents a sealing plug which may, for example, be made ofrubber. Connected to match head l9 and extending through plug 29 are legwires 3| and 33 covered by insulation 35 and 37 respectively. Alsoextending through plug 29 is wire 39 covered with insulation 43. It willbe noted that wire 39 is bent around the bottom of plug 29 so as toinsure its being in contact with shell II. The ends of leg wires 3i and33 and wire 39 are connected together by means of a metal shunt 4|.

Any suitable means may be employed to shunt the three wires, 3|, 33, and39, together. Merely twisting together is usually sufiicient; however,it is generally found convenient to use a metal shunt.

In the cap of the invention the electrical connection between the firingcircuit and the cap shell provides static protection such as may beobtained by grounding one leg wire against the shell. However, since thewire which is connected to the shell may be disconnected from the firingcircuit when the initiator is to be fired no problem of current leakagesuch as exists with initiators in which the firing circuit is groundedto the shell exists.

Also the fact that the shell is connected directly with the shunted endof the leg wires in accordance with the present invention the straycurrent danger is very considerably reduced. While an initiator having agrounded firing circuit and shunted leg wires offers considerableprotection against stray currents, there is al- Ways the possibilitythat a stray current reaching the firing circuit from the initiatorshell may divide and go partially through the ignition composition anddetonate the cap. When the shell is connected directly to the shunt andthe firing circuit insulated from the shell except through the shunt, asin the present invention, no such division in current is possible.

Initiators of the present invention will also be found to beparticularly valuable in testing initiators for grounded firing circuitsby connecting ablasting galvanometer between the firing circuit and theshell wire. Occasionally in manufacture or handling, electric explosioninitiator firing circuits become grounded to the shell walls and it isdesirable to have a ready means for testing for such grounds. Inordinary use of the initiators of the present invention the shell wireis maintained connected to the leg wires until the initiator is to beconnected to the firing means or to be tested. When an initiator is tobe used in series with several other initiators it is desirable that theshell wires of all the initiators be connected together or to a commontrunk line, and the common trunk line kept connected to the leads to theseries until final firing connections are made.

In this way if a series of electric explosion initiators must be leftfor a period of time such as overnight and exposed to electrical storms,the firing circuit and the shell walls are maintained so that one cannotbecome charged to a different potential than the other.

The time of firing of the electric explosion initiators of the presentinvention may readily be determined. This feature is of particularadvantage in seismograph work. To determine the time of firing apotential is imposed across a wire of the firing circuit and the shellwire. Since there is no connection in the cap between the shell and thefiring circuit no current will ordinarily pass. However, when theinitiator ex- 4 plodes ionization takes place in the cap and will permitcurrent to pass between the firing circuit and the shell. Passage ofthis current can be recorded by suitable means, for example anoscillograph, to record the time of firing of the initiator.

It is desirable but not necessary that the shell wire be insulated in asimilar manner to the leg wires, and it is also desirable that the shellwire be marked so that it will not be confused with the usual leg wires.This may be suitably done by providing the shell wire of a differentsize or with insulation of a different color.

Conveniently the shell wire and the two leg wires may be bound togetheror in the case of plastic wire extruded together to provide a convenientor rugged assembly.

If it is desirable to use the third wire, other than the leg wires,merely for the purpose of determining the time of firing of theinitiator as outlined above, it is not necessary that the third wire bein electrical contact with the shell; it may simply terminate inside theinitiator so as to provide a pole for passage of current through theionized atmosphere inside the cap. However, when the wire is connectedwith the initiator shell, as is preferred, a larger contact is of courseprovided.

What is claimed is:

1. An electric explosion initiator comprising a metal shell, anexplosive composition, a. firing circuit including two insulated legwires electrically connected in the shell by a bridge wire, and a thirdwire leading into the initiator and being electrically insulated fromthe firing circuit in the initiator, said three wires being electricallyconnected outside of said initiator by a removable shunt path of lowresistance.

2. An explosion initiator comprising a metal shell, an explosivecomposition, a firing circuit including two insulated leg wires, saidfiring circuit being electrically insulated in the initiator from saidshell, and a third wire connected to said metal shell, the said threewires being electrically connected outside of said initiator by aremovable shunt path of low resistance.

3. An explosion initiator comprising a metal shell, an explosivecomposition, a firing circuit including two insulated leg wires, saidfiring circuit being electrically insulated in the initiator from saidshell, and a third wire leading into the initiator and connected to saidshell, the said three wires being electrically connected outside of saidinitiator by a removable shunt path of low resistance.

DAVID M. MCFARLAND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,597,445 Grant Aug. 24, 19262,189,741 Minton Feb. 6, 1940 2,408,125 Rolfes Sept. 24, 1946 I FOREIGNPATENTS Number Country Date 2,826 Great Britain of 1881 3,659 GreatBritain of 1879 534,689 Great Britain Mar. 14, 194i

